Nonvolatile memory devices retain data written in their cells even when their power supplies are interrupted. The nonvolatile memory devices include flash memory devices. Since flash memory devices can perform electrically batch-erasing operation for data of their cells, they have been widely used in computers and memory cards.
Flash memory devices may be classified into NOR flash memory devices and NAND flash memory devices according to the connection states of cells and bitlines. NOR flash memory devices can easily cope with the speeding up of operations while being disadvantageous in high integration due to their high power consumption. On the other hand, NAND flash memory devices are advantageous in high integration due to their lower consumption of cell current than NOR flash memory devices.